Gallium nitride (GaN) based light emitting diodes (LEDs) have been used in a wide range of applications including natural color LED displays, LED traffic lights, white LEDs, etc. In recent years, a highly efficient white LED is expected to replace fluorescent lamps and, in particular, efficiency of the white LED approaches efficiency of typical fluorescent lamps.
The GaN-based LED is generally formed by growing epitaxial layers on a substrate, for example, a sapphire substrate, and includes an n-type semiconductor layer, a p-type semiconductor layer and an active layer interposed between the n-type semiconductor layer and the p-type semiconductor layer. Further, an n-electrode pad is formed on the n-type semiconductor layer and a p-electrode pad is formed on the p-type semiconductor layer. The LED is electrically connected to and operated by an external power source through these electrode pads. Here, electric current is directed from the p-electrode pad to the n-electrode pad through the semiconductor layers.
To assist current spreading in the LED, the LED includes extensions extending from the electrode pads. For example, U.S. Pat. No. 6,650,018 discloses an LED which includes a plurality of extensions extending in opposite directions from electrode contact portions, that is, electrode pads to enhance current spreading. The use of the extensions extending from the electrode pads may result in improvement in efficiency of the LED through current spreading.
However, an n-electrode pad and n-electrode extensions are generally formed on the n-type semiconductor layer exposed by etching the p-type semiconductor layer and the active layer. Accordingly, the formation of the n-electrode pad and the n-electrode extensions results in a reduction in light emitting area, causing deterioration in light emitting efficiency.
Meanwhile, since the electrode pads and the electrode extensions are formed of metal, the electrode pads and the electrode extensions absorb light generated in the active layer, thereby causing optical loss. Further, although the use of the electrode extensions enhances current spreading, current crowding still occurs at regions near the electrode extensions, causing electrode extension induced optical loss. In addition, since the electrode pads and the electrode extensions use a material such as Cr, which exhibits low reflectivity, as an underlying layer, optical loss becomes severe due to optical absorption by bottom portions of the electrode pads and/or the electrode extensions.
Furthermore, as the size of the LED increases, the likelihood of a defect being present in the LED increases. For example, defects such as threading dislocations and pin-holes, provide a path through which electric current flows rapidly, thereby disturbing current spreading in the LED
Moreover, when a large LED of 1 mm2 is operated at a current of about 200 mA or more, the current crowds through such defects or through a certain position and the LED suffers a severe reduction in external quantum efficiency relating to current density, which is referred to as a droop phenomenon.